Field of the Invention
The present invention relates to an oxygen concentration measuring device for determining the air-fuel ratio of an air-fuel mixture before burning by measuring the oxygen concentration, etc., in exhaust gas from an internal combustion engine, and in particular to an oxygen concentration measuring device suitable for measuring oxygen concentration by using a limit current type oxygen sensor.
Description of the Related Art
Heretofore, in this type of oxygen concentration measuring devices, as indicated e.g. in JP-A-59-163556, paying attention to the fact that the internal resistance of the limit current type oxygen sensor varies depending on the temperature thereof and that a current-voltage characteristic curve specifying the temperature, i.e. the internal resistance, of the oxygen sensor passes through the origin, the oxygen sensor is positively biased in a first period by means of a positive voltage source, while it is negatively biased in a second period by means of a negative voltage source. Current flowing through the oxygen sensor is detected in the first and the second periods. The oxygen concentration is determined on the basis of the current detected in the first period, while the internal resistance of the oxygen sensor is determined on the basis of the current detected in the second period. In this way, the air-fuel ratio is reliably measured on the basis of the oxygen concentration thus detected, while controlling the temperature of the oxygen sensor at a high level of precision so as to maintain the sensor within an active region.
On the other hand, as indicated in JP-B-1-28905 and JP-B-1-25419, paying attention to the fact that the internal resistance and the temperature of the oxygen sensor correspond to each other in a one-to-one relationship, the restriction on the usable temperature region and the region where oxygen concentration can be measured is removed by detecting the internal resistance of the oxygen sensor, by calculating the voltage applied to the oxygen sensor, starting from the detection value thus obtained, and applying the voltage to the oxygen sensor on the basis of the calculation.
However, in such a construction, because the second period described previously is repeated uniformly, even in a state where the air-fuel ratio can be measured stably, the air-fuel ratio can be measured only after the second period has lapsed, which gives rise to an inconvenience that the period in which the air-fuel ratio can be measured is retarded. Further, since the second period described above is set repeatedly, independently from a temperature decrease of the oxygen sensor, the period in which the negative voltage is applied to the oxygen sensor is not always in accordance with a point of time where the temperature of the oxygen sensor begins to decrease. Consequently, if the temperature of the oxygen sensor is lowered too much before the period in which the negative voltage is applied and after the state where the air-fuel ratio can be stably measured has-been once established, a long period of time is required before the air-fuel ratio can be measured stably because the response to the temperature of the oxygen sensor is slow, even if the temperature control as described above is effected by applying the negative voltage. In such a case, because the second period described above is set at a period of time required for stabilizing the internal resistance of the oxygen sensor, this causes the period in which the air-fuel ratio can be stably measured to be further retarded.
Furthermore, because the voltage applied to the oxygen sensor is continuously varied corresponding to variations in the internal resistance of the oxygen sensor, the amount of data is increased extremely. On the other hand, if data are interpolated in order to decrease the amount of data, measurement precision of the internal resistance is reduced correspondingly and, in addition, a period of time for measuring the internal resistance is increased.